The present invention relates to semiconductor lasers and detectors for detecting light from a semiconductor laser.
A semiconductor laser is a device which emits light of substantially a single wavelength. The light from this type of laser can be focused to a spot with a diameter comparable to its wavelength. The semiconductor laser belongs to the same family of semiconductor devices as the LED (light emitting diode). However, the light from an LED has a broader spectrum of wavelengths and thus cannot be focused as sharply as a semiconductor laser. The structure and composition of the laser determine its wavelength, its expected lifetime and its light guiding mechanism. Examples of laser structures are gain guided lasers and index guided lasers. Examples of laser structures are shown in U.S. Pat. Nos. 3,479,613, 3,457,468, 3,293,513, 3,257,626, and 4,483,480.
Most commercial semiconductor lasers contain a photodetector behind the lasing structure to monitor the level of light emitted at the front of the laser chip. The monitoring photodetector provides a signal which is used to maintain a constant laser output. It has been noted in some applications that when a small amount of light emitted from the laser is fed back into the laser chip, it causes a change in the laser output power which can be detected by the rear photodetector. Otherwise, most applications use a separate photodetector to monitor light reflected off of an object by a laser. This light is typically supplied to the separate photodetector through a beam splitter or other device.
The light emitted from a standard laser diode comes from an edge of the laser chip, and thus is emitted parallel to a top surface of the semiconductor chip. The parent patent application referenced above showed a semiconductor laser and photodetector mounted within a single housing. FIG. 1A shows a top view of such a hybrid device. A number of connector pins 1-10 are shown which extend through the bottom of the container. A laser diode chip 12 is bonded to a silicon substrate 11 which is in turn bonded to a first, side surface 16 of a heat sink 14. A four quadrant photodetector 13 is bonded to a second, top surface 15 of heat sink 14. The light emitted by laser diode 12 comes out of the page in FIG. 1A, is reflected off of a medium and directed back to photodetector 13.
FIG. 1B shows a side view of the hybrid device in FIG. 1A. During manufacturing, laser chip 12 is provided with wire bonding connections to pins 2 and 4 with a device in the orientation shown in FIG. 1B. The device is then rotated by 90 degrees to the orientation in FIG. 1A to connect the four quadrant photodetector to the remaining pins.
A recent advance in semiconductor laser diodes is the development of a surface emitting laser. FIGS. 2A, 2B and 2C show three embodiments of such surface emitting lasers. In each case, the laser light is emitted from a top surface of the semiconductor laser diode, rather than a side or edge surface. In the embodiment of FIG. 2B, for example, this is done by cutting a notch in the device and using a reflective mirror to direct upward the light being emitted to a side. Such surface emitting lasers are discussed in an article entitled "R & D on Surface-Emitting Diode Lasers", James N. Walpole, Laser Focus/Electro-Optics, Page 66-70 (September, 1987).